96 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



but pass from fine, through coarse, into fine again, when M moves, indicating 

 that the horizontal minor axes of the ellipses now occupy the middle of the 

 field of the telescope. Scarcely any curvature of the lines can now be seen. 

 In the first position (top of ellipses visible), o.oooi cm. produces very visible 

 change of inclination of the nearly horizontal lines. 



With the very fine lines even this adjustment fails to bring the center of 

 ellipses into the field. Thus, in the compensated case, 2+4, with M at 2.025 

 cm., the spot of light on the opaque mirror N may (with simultaneous incli- 

 nation of the grating) be moved fully 10 cm. vertically, without producing 

 marked effect on the interference pattern. The lower parts of enormous 

 circles are intersected by the field of the telescope throughout. 



58. Film or ruling on one side of the glass plate. Ruled grating, If the 



film is on one side only, the case of the plate-glass grating is reproduced. From 

 the absence of so many superposed spectra, the results should therefore be 

 better. 



The plate-glass grating, 10,000 lines to the inch, mounted in the interferom- 

 eter with its ruled face toward the collimator, shows but two lines for the 

 reflection from each of the opaque mirrors, of which three only give rise to 

 spectra, the beam of the fourth being reflected from the rear face and not 

 passing through the grating, after reflection. But this case may be reached 

 by reversing the grating (ruled face toward the telescope), under which con- 

 ditions all the rays have spectra. The micrometer positions for these inter- 

 ferences were as follows: 



M at 2.95 cm. Ruled face in front. Reflection from the front face. Path 

 difference 5. Reflection from the rear face may be screened off at the opaque 

 mirror N. Fine, sharp, solitary ellipses, very black lines on an even, brilliantly 

 colored ground. No stationary interferences from the front and rear faces of 

 the grating. 



M at 1.90 cm. Ruled face front or rear. Reflection from front and rear 

 faces conjointly. Compensated position. Path difference 5 = o. Fine revolv- 

 ing hair-lines belonging to a remote center. 



M at 0.85 cm. Ruled face to the rear. Path difference 8. Fine ellipses, 

 but complicated by stationary interferences and superposed spectra, which 

 can not be easily screened off. The first position is here also available. 



The position (2.95 cm.) with ruled face toward the collimator, is thus the 

 practical case, since it is sharpest and without complications. 



The micrometer displacement A./V, which should correspond to the extreme 

 positions of the grating (ruled face forward and ruled face rearward), if e is 

 its thickness, / and R the angles of incidence and refraction, M the index of 

 refraction, is 



cos R 



Here roughly 7=15; ^ = 9.7; M=i-53J = 0.67 cm. Hence 



AAf= 2X0. 67X1. 53X0. 986 = 2.02 cm. 



